1. Field of the Invention
The present invention relates to a sealing device used at a seal portion of a member for shielding one gas atmosphere from the other gas atmosphere and, more particularly, to a sealing device used at a coupling portion of a wall for shielding the process atmosphere of a heat treatment apparatus from the open air.
2. Description of the Related Art
In an early operating state, the internal pressure of a process tube of a CVD apparatus is normally set to be smaller than 10.sup.-3 Torr. For this reason, a furnace wall coupling of the CVD apparatus has a seal structure constituted by a seal member such as an O-ring so as to prevent the open air from entering the process atmosphere through a slight gap in the coupling.
A sealing mechanism constituted by a combination of seal rings and an evacuation groove is disclosed in "Design parameters for differentially pumped rotating platforms", Rev. Sci. Instrum. 58(2), February 1987, p. 309, p. 310, U.S. Pat. No. 4,726,689.
As shown in FIG. 1, the above-mentioned sealing mechanism is formed by assembling coupling flanges 2 and 3. A plurality of seal rings 4 are inserted between flange 2 and flange 3, while a intermediate space 5 is formed between one seal ring 4 and other seal ring 4. Each seal ring consists of a resin member incorporating a spring. The intermediate space 5 is formed to be coaxial and parallel with the seal rings 4 and communicates with a suction portion of a vacuum pump through a path 6. In this sealing mechanism, a seal effect is obtained from a synergistic effect combined of the shielding effect by seal rings 4 and the evacuation effect by pumping through the intermediate space 5.
The seal ring 4 of such a type, however, is generally composed of a flexible fluororubber or an ethylene fluoride resin. These materials have heat resistance temperatures of about 200.degree. C. at best. Therefore, if the operation temperature of the heat treatment apparatus exceeds the heat resistance temperature of the seal ring 4, the seal ring 4 is thermally deformed. As a result, a desired seal effect cannot be obtained. For this reason, portions near the seal rings 4 must be forcibly cooled to protect them.
In a CVD apparatus, if an O-ring (seal ring) coupling portion is forcibly cooled, deposits (reacted product) of process gas components are produced on an inner wall portion of a process tube near the coupling, resulting in a nonuniform temperature distribution in the process tube. In order to prevent the cooled coupling portion from influencing a uniform temperature zone, the process tube must be elongated. This undesirably increases the longitudinal size of the CVD apparatus.
In addition, each O-ring is set in a semi-molten state by heating during an operation. When the CVD apparatus is stopped to replace the O-ring with a new one, since the O-ring adheres to the contact face of flange, it is difficult to detach the flanges from each other. When this O-ring is detached, a quartz process tube may be damaged.
Furthermore, in an initial stage in the use of the sealing device, when the O-rings are heated, moisture and components contained in the O-rings are evaporated as gases. The gases and moisture evaporated from the O-ring adversely affect the atmosphere in the process tube to cause variations in quality among process lots.
Moreover, in a reduction heat treatment for removing natural oxide films from semiconductor wafers by using process gases such as monosilane (SiH.sub.4), even if the process tube is sufficiently evacuated by using a vacuum pump, gases and moisture evaporated from the O-rings cannot be neglected. This makes it difficult to perform a desired treatment of the semiconductor wafers.
In addition, assume that evacuation is performed by using both a mechanical pump and an ion pump. In this case, since the ion pump functions from the order of 10.sup.-3 Torr, roughing is performed first by the mechanical ion pump. Therefore, a relatively long period of time is required to obtain a seal effect.